Flat display panels have become the mainstream products in the current display panels because of the advantages such as lightness, portability, low power consumption and so on. The flat display panel is, for example, an organic light-emitting diode (OLED) display panel a liquid crystal display (LCD) panel or the like. For example, the OLED display panel is mostly an active matrix (AM) type display panel, which uses a thin film transistor (TFT) and a capacitor to drive the OLED to emit light. The thin film transistor connected to the OLED and controlling a current flowing through the OLED is generally called a driving transistor, and the stability of the threshold voltage of the driving transistor has an important influence on the display quality of a display panel.